SUMMARY Many studies support a role for IL-23 in the pathogenesis of IBD, but it is unclear how IL-23 expression in vivo promotes colitis. To better study the biology of IL-23 in immune-competent mice we have developed a mouse model (R23FR mice) in which expression of IL-23 is promoted at low levels in CX3CR1+ cells in the intestine, upon administration of tamoxifen (TAM). IL-23 induction via repeated cycles of TAM dissolved in a diet (diet 2019) different from the diet used in our facility (diet 5053) led to development of marked colitis that resembled ulcerative colitis in humans. Cycles of relapse (flares) and remission, observed in humans, were also observed in these animals upon diet switch. Mechanistic studies demonstrated that the diet switch promoted marked changes in the microbiota of R23FR mice and their littermate controls (FR mice). Colitis induction and flares required the presence of the microbiota and CD4+ T cells. CD4+ T cells from the mesenteric LN or colon of R23FR mice at remission, but not those of controls, transferred disease to Rag-/- mice, but only when the recipient mice received diet 2019, suggesting previous priming of the CD4+ T cells. Single cell transcriptomic analysis of the disease-inducing CD4+ T cells demonstrated the existence of distinct populations expressing the cytokines IL-17, IL-22, IFN?, chemokines, and cytotoxic molecules. Gene-deficient mice and antibody blockade showed that IL-22 and IL-17 were not critical for disease development, suggesting the existence of alternative effector mechanisms. Indeed, in vitro experiments showed that CD4+ T cells from R23FR mice can directly kill intestinal epithelial cells. Finally, MHC-II expression by epithelial cells was required for disease development. Using this novel mouse model we will test the general hypothesis that changes in diet occurring simultaneously with low level expression of IL-23 result in an immune response to the commensal microbiota or their products. In Aim 1 we will define how diet changes and IL-23 expression promote immune priming. In Aim 2 we will define how diet changes and IL-23 expression promote effector immune responses.